Metallic articles are fabricated by any of a number of techniques, as may be appropriate for the nature of the metal and the article. In one common approach, metal-containing ores are refined to produce a metal. The metal may be further refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or after solidification and remelting. After a metal of the desired composition is produced, it may be used in the as-cast form for some alloy compositions (i.e., cast alloys), or further worked to form the metal to the desired shape for other alloy compositions (i.e., wrought alloys). In either case, further processing such as heat treating, machining, surface coating, and the like may be employed.
Some of the most demanding applications of materials are in aircraft gas turbine engines. Some examples of materials applications in gas turbine engines include turbine disks made of nickel-base alloys, combustor liners made of cobalt-base alloys, and stationary high-temperature seals made of iron-base alloys. The materials of construction of these components must exhibit the required mechanical properties under these operating conditions.
These components and other articles are typically manufactured by furnishing the metallic constituents of the selected alloy, melting the constituents, and casting the molten mixture into a crucible to form a cast ingot. For some alloy compositions and types of articles, the cast material is used in the cast form. For other alloy compositions and types of articles, the cast ingot is mechanically worked, first by converting it into a billet. The billet is further mechanically worked, typically by forging, rolling, extrusion, or the like, to its final form, and then machined to produce the final cast-and-wrought component.
Small mechanical or chemical defects in the article may cause the article to fail prematurely in service. Mechanical defects include, for example, cracks, voids, ceramic particles that are present from the melting crucible, or dross floating on the surface of the melt. Chemical defects include, for example, elemental segregation that occurs during solidification or undesired chemical species that result from chemical reactions between elements present during the melting process. Both mechanical and chemical defects may cause cracks to form prematurely in engine service. A failure resulting from these defects may be catastrophic to the gas turbine engine and possibly to the aircraft. The manufacturing process must also produce a microstructure in the final article that exhibits the desired combination of mechanical properties and physical properties required in the components.
It has been possible, using existing melting, casting, and conversion practice, to reduce the presence and size of chemical defects in installed components to reasonably low levels. However, there is always a desire and need for a manufacturing process to produce the components with a further reduction in the incidence of such chemical defects, thereby improving the operating margins of safety. The present invention fulfills this need for an improved process, and further provides related advantages.